SI Units is a internationally recognized system of units of measurements used by not just chemists, but almost all scientists alike. The Systeme International d’Unites (SI Units) provide a standardized way of quantifying measurements. There are a total of seven SI Base Units. All of the SI base units are defined by a physical constant, with the exception of kg. Here are the seven SI base units. It is beneficial for you to familiarize yourself with these units as they are trivial and sometimes appear on olympiads and standardized tests.
| Physical Quantity | Symbol | Base unit | Unit Symbol |
|---|---|---|---|
| Mass | \(m\) | kilogram | \(kg\) |
| Length | \(l\) | metre | m |
| Time | \(t\) | second | s |
| Current | \(I\) | ampere | A |
| Temperature | \(T\) | kelvin | K |
| Amount of substance | \(n\) | mole | mol |
| Luminous intensity | \(I_v\) | candela | cd |
Table adapted from Housecroft Chemistry 4th Ed. One needs to be wary that physical constants change when units of variables in the equation change. For example, in the ideal gas law equation,\[Pv=nRT\] is the gas constant, and is given a value of 8.314. This is only the case,however, when the unit of P, pressure, is in Pascals. If it is in Atomosphere (another unit for pressure), R changes to a completely different magnitude. Now that we have covered the seven fundamental SI units, it is time to move on to the derived units. The SI base unit system is perfectly capable of quantifying every single measurement there is to quantify (within the scope of human knowledge), but for the sake of convenience and user-friendliness, scientists have derived other units, such an action becomes more apparent as you will see as writing everything in terms of SI base units are very tedious in almost all the cases. Here is a table of SI derived units that we primarily deal with in the study of chemistry.
| Unit | Name of Unit | Symbol | Relation to base |
|---|---|---|---|
| Energy | joule | J | \(kgm^2s^{-2}\) |
| Pressure | pascal | Pa | \(kgm^{-1}s^{-1}\) |
| Electric Charge | coulomb | C | \(kgms^{-2}\) |
| Electromotive Force | volt | V | \(As\) |
| Resistance | ohm | \(\Omega\) | \(A^2s^4kg^{-1}m^{-2}\) |
If you are not convinced about this, here is an example:
\[Ek=12mv^2\]
The above equation is the equation to calculate the kinetic energy of an object in motion, given its mass and velocity, both of which are in SI base units, (mass in kg, and velocity in ms\(^-1\)). We know that the unit for energy is in joules so if we put in some units and do our algebra, we end up with this:
\[E_k=12(kg)(ms^{−1})2\]\[E_k=kgm^2s^{−2}\]
Note that because 1/2 is just a number, we can eliminate it as it is unitless and dimensionless. If you refer to the table above, you would notice that the unit we ended up with is exactly joules! This applies for all SI derived units as they can be derived from the 7 SI base units. This property of the SI units is referred to as Self Consistency. In the study of chemistry, we are primarily concerned with extremely small things, down to the molecular level. However, most of the SI units were meant to quantify things that are tangiblly sized in real life. The mass of a proton in just kg would be 0.0000000000000000000000000001673 kg. This is especially inconvenient as a way of notating the mass of a proton. Let alone an electron! Scientists have figured out a way around this by implementing a standardized system of nomenclature referred to as The Scientific Notation, where instead of writing 27 zeros, you can simply write \(\times{}10^{-27}\). So the mass of the proton is \(1.673\times{}10^{-27}\). In addition, scientists have also implemented what is called the SI multipliers/prefixes. When was the last time you heard someone say:”This room is 100000000 centimeter squared”? Sound absurd doesn’t it? It is much easier and common to say, 100 meter squared. Refer to this chart below for reference .